Viscometers are an important tool in characterizing the rheological properties of products in industries such as food processing [1], consumer products [2], inks [3], polymers [4], drilling fluids [5] and lubricants [6]. In most of these industries, there is often a need to quickly characterize viscosity of one or more samples at the location where products are being made or processed, rather than being shipped offsite. Mechanical rheometers although capable of performing a broad suite of precise rheological measurements are not well suited for such onsite or field viscometry applications, because of their bulkiness and need for skilled operators. Viscosity measurement devices such as Saybolt, capillary tube and rotational viscometers are more suited for onsite usage because of their non-complicated operation and cost effectiveness. Although handy, these devices have a number of limitations which include (i) use of large sample volumes (ii) cumbersome cleaning procedures if multiple sample measurements are needed (iii) limited shear rate range and (iv) the presence of non-viscometric flow kinematics (e.g. Saybolt viscometer) making it difficult to interpret viscosity data for complex fluids.
In the last decade, microfluidic viscometers [7-13, 17] have emerged as alternative tools capable of addressing the above limitations of conventional viscometers. Microfluidic viscometers developed to date use a variety of driving sources to introduce fluid flow in microchannels. In some cases constant fluid flow rate is imposed using syringe pumps [7, 10-12], in other cases constant pressure drop is delivered using pressure sources [13] or capillary pressure gradients [9]. Knowing the relation between pressure drop and flow rate, the viscosity of the fluid is determined. In these devices, depending on the driving force, the fluid response is measured using pressure sensors embedded on the channel surface [7, 12] or image-based detection of fluid interfaces [8-10, 13, 14, 17].
Despite several microfluidic viscometers being reported in the literature, current devices have some limitations. For example, in pressure-sensor based viscometers because the sensing is coupled to fluid flow, they are not ideal for handling clinical samples where use-and-throw capability is desired to avoid sample cross-contamination. Likewise, repeated handling of industrial-grade particulate fluids in these devices may become problematic due to adhesion of particles on channel and sensor surfaces, unless rigorous washing protocols are implemented. Finally, pressure-sensor based devices do not scale favorably for parallelized measurements. In contrast, microfluidic viscometers based on imaging sensors have the advantage that the element is un-coupled from fluid flow in the channel. This feature not only allows parallel of multiple samples, but also creates the opportunity to fabricate disposable devices. Current approaches to image-based viscometry rely on the use of research-grade microscopes and cameras, making them only suitable for laboratory environments. However, such approaches are not simple and flexible enough for onsite or field applications in resource-poor settings.
Rheosense has a device in market that uses pressure sensors embedded in microfluidic chips to find viscosity of fluids. Anton Paar and AR instruments have rheometers in the market, which are mechanical in nature and use rotors to imply stress on fluids and calculate the corresponding viscosity.